Coating of tablets

ABSTRACT

Tablets are coated by tumbling a batch of tablets in a rotating coating pan and subjecting the tablets while they are in the pan to a plurality of treatment steps. Each of these treatment steps consists of a plurality of treatment cycles each including a dosing period in which a measured quantity of tablet coating liquid is introduced into the pan, a drying period in which drying air is supplied to the interior of the pan and an intermediate mixing period during which the tablets tumble in contact with the already introduced coating liquid for a pre-set time and no drying air is supplied to the pan. The temperature of the batch of tablets being coated in the coating pan is sensed by a sensor. The temperature of the batch of tablets at the end of a pre-set time interval from the commencement of the drying period is stored in a memory device as a reference temperature. Whenever during any one cycle there is a preselected difference between the sensed temperature of tablets in the pan and the reference temperature stored in the memory devices a signal is produced. This signal is utilized to terminate the drying period of such cycle and to initiate the dosing period of the next cycle.

This is a division of application Ser. No. 595,128, filed July 11, 1975,now U.S. Pat. No. 4,168,674.

BACKGROUND OF THE INVENTION

This invention relates to the coating of tablets. It is to be understoodthat the expression "tablets" as used herein is intended to refer solelyto articles having a tablet-like shape which can be tumbled in a coatingpan and includes articles having a ball-like, semi-spherical,torpedo-like or other shape. The definition is not restricted topharmaceutical dosage forms, but may relate to articles coated in theconfectionary or other industries.

In the pharmaceutical industry it is well known to provide a core tabletcontaining a medicament with a sugar coating by tumbling a quantity ofcore tablets in a pan which rotates about an axis which may be inclinedwith respect to the horizontal. A coating liquid, which may be asolution or suspension, is applied to the mass of tablets in the pan andas the pan rotates the surface of the tablets become evenly coated bytheir tumbling action in contact with the coating liquid. Theapplication is then dried in an air stream. In general, a large numberof separate applications is necessary before a satisfactory tablet isfinally produced and whilst many of these applications are identical, acoating method may also include applications of different volumes or ofdifferent coating liquids. A similar technique is used for themanufacture of some sweetmeats.

The coating of tablets in this known way is not easy to control andrequires skilled operators giving constant attention, especially whenworking to the high standards of quality control demanded by thepharmaceutical industry. Hence the process is expensive in terms of timeand labour.

An object of the invention is to simplify and standardise the coating oftablets and provide a more efficient coating method in terms of time,resources and quality of finished product as compared with knownmethods.

BRIEF SUMMARY OF THE PRESENT INVENTION

According to the present invention there is provided, a method ofcoating tablets comprising the steps of tumbling a batch of tablets in arotating coating pan and subjecting the tablets while they are in thepan to a plurality of treatment steps each of which consists of aplurality of treatment cycles each including a dosing period in which ameasured quantity of tablet-coating liquid is introduced into the pan, adrying period in which drying air is supplied to the interior of the panand an intermediate mixing period during which the tablets tumble incontact with the already introduced coating liquid for a pre-set timeand no drying air is supplied to the pan, sensing the temperature of thebatch of tablets being coated in the coating pan, storing in a memorydevice as a reference temperature the temperature of the batch oftablets at the end of a pre-set time interval from the commencement ofthe drying period, continuing the sensing of the temperature of thetablets in the pan and producing a signal whenever during any one cyclethere is a preselected difference between the sensed temperature oftablets in the pan and the reference temperature stored in the memorydevice and utilising such signal to terminate the drying period of suchcycle and to initiate the dosing period of the next cycle.

The invention also provides an apparatus for coating tablets comprisinga rotatable coating pan capable of receiving a batch of tablets to becoated, means for supplying coating liquid to a batch of tablets insidethe pan, means for supplying drying air to the interior of the pan, anda control unit operable to control the operation of both of the supplymeans during each of a plurality of treatment cycles each of whichincludes a dosing period in which a measured quantity of liquid issupplied to the pan, a drying period in which drying air is supplied tothe interior of the pan and an intermediate period during which thetablets tumble in the pan in contact with the already applied liquid fora pre-set time and no drying air is supplied to the pan; the saidcontrol means including means automatically operable when the last ofone pre-set number of cycles has been completed to initiate operation ofboth supply means for another preset number of cycles in which the sameor different measured quantities of the same or a different liquid aresupplied, a temperature detector arranged to sense and respond to thetemperature of a batch of tablets in the pan, a memory device in whichis stored information as to a reference temperature which is thetemperature of the batch of tablets in the pan determined by thedetector at the end of a pre-set time interval from the commencement ofthe drying period, and means responsive to a preselected differencebetween the temperature sensed by the detector and the said referencetemperature stored in the memory device to terminate the drying periodof the cycle in which the difference occurs and to initiate the dosingperiod of the next cycle.

BRIEF DESCRIPTION OF THE DRAWINGS

In the accompanying drawings:

FIG. 1 is a schematic diagram of a tablet coating apparatus, and

FIG. 2 is a block function diagram.

DESCRIPTION OF PREFERRED EMBODIMENTS

In the illustrated embodiment of the invention an apparatus for coatingtablets comprises an open coating pan 1 which is rotatable by anelectric motor (not illustrated) about an axis which is inclined withrespect to the horizontal. Core tablets to be coated are placed in thepan and are then tumbled by rotation of the pan. A pre-selected volumeof a coating liquid is pumped into the pan 1 by a peristaltic or othersuitable pump 2. The volume of coating liquid pumped into the pandepends on the nature of the liquid to be applied, the stage of thecoating operation and the quantity of tablets in the pan. After thetablets have been tumbled in the coating liquid for a pre-determinedtime hot air is blown through a duct 3 on to the tablets while they arebeing tumbled to dry the surfaces of the tablets. The hot air supply iscontrolled by a butterfly valve 4 in the hot air duct 3 which iscontinuously supplied with hot air from any suitable source 5 such as aheat exchanger and fan. An extractor duct 3E connected to an extractorfan (not shown) withdraws air from the interior of the pan 1.

Such a coating operation is known. The present invention provides ameans for controlling the supply of coating liquid and the applicationof drying air to a reproducible standard.

The control means 6 is illustrated in detail in FIG. 2 and comprises asheet metal housing at the front face of which is a control panel. Thehousing contains a plurality of adjustable electronic counting devices7a and 7b and electronic or mechanical timers together with a pulsegenerator which operate an arrangement of relays and switches in a unit9 to switch the coating liquid supply pump on and off as desired and toopen or close the valve 4 controlling the application of hot drying airto the interior of the pan 1. The counting devices 7 are arranged inpairs (the counters of each pair being designated "a" and "b" in asequence of channels to provide a variable pre-set number of dosageapplications of different pre-set volumes and of different coatingliquids if required. All the counting devices 7 are adjustable and thefirst one of each pair may be pre-set to produce a desired number ofcycles. The second counting device 7b each pair controls the volume ofcoating liquid to be applied during each cycle of the channel containingthat pair by connecting to a pulsating flow meter 10 located in thecoating liquid feed line 11. Each cycle includes three periods, namely adosing period in which the pump 2 is switched on to pump coating liquidinto a predetermined position or positions in the pan 1, andintermediate delay or mixing period in which the tablets are tumbled inthe pan 1 after the dosage application and which is adjustable by meansof a panel-mounted timer 12, and a drying period in which the valve 4 tothe hot air supply duct 3 is automatically opened to permit drying airto be blown across the surface of the mass of tablets tumbling in thepan 1.

When the drying period of the last cycle in a particular channel hasbeen completed the first counter 7a of the next channel is switched into control a further number of cycles. The number of cycles controlledby this next counter 7a may be pre-set to the same as in the previouschannel or the number can be different, but generally the secondcounting device 7b in this next channel is pre-set to deliver adifferent volume of coating liquid from that supplied by the firstchannel by counting a different number of volumetric pulses from theflow meter 10 before terminating the application of coating liquid. Theintermediate mixing period and the drying period then proceed as beforeaccording to the mixing time initially pre-set, and the drying controlconditions as hereinafter described. When the drying period of the lastcycle of the last channel in the unit is completed the supply of bothcoating liquid and drying air is terminated.

A number of channels are operable to connect to the pump 2 a reservoir13 of coating liquid associated with those channels. On passing from thelast of these channels to the first of the next an alternative reservoir14 may be automatically connected by operation of a changeover valve 15to provide a different coating liquid for those channels.

The drying period of each cycle is controlled in accordance with thetemperature of the tablets being tumbled. For this purpose a secondstage control unit is associated with the first stage unit and is housedin the same sheet metal casing. This second stage control unit includesa thermocouple 16 or other temperature sensor which is inserted into themass M of tablets in the pan 1, usually at a predetermined position ofadequate sensitivity. The thermocouple 16 supplies to the second stagecontrol unit a signal indicative of the temperature of the tablets beingtumbled. The second stage control device includes a memory device 17/18which stores information as to a reference temperature of the mass orbatch M of tablets during the hot air application, this referencetemperature being the temperature of the mass of tablets at the end of apre-set time interval from the commencement of the drying period. Thiscontrol unit is switched in by the first stage control unit at thedrying period of the cycles controlled by the first stage unit. Once thesecond stage unit has been switched in it will on receipt of a signalfrom a temperature difference analyser 18 indicative of the attainmentof a pre-selected difference in temperature (conveniently in the range0.3° C. to 3° C.) of the tablets in the pan 1 from the referencetemperature already stored in the memory device 17/18 operate to send acommand signal to the first stage unit to terminate the drying period ofthat particular cycle so that a new cycle may be initiated. The controlunits are therefore ultimately concerned with temperature differencemeasurement, and hence moisture content, to give coated tablets ofreproducible quality. The coating is not dependent on time since cycletimes may vary with slight variation in drying conditions, and is notdependent on fixed temperature which cannot be related to residualmoisture content of the mass of tablets.

The various periods in the cycles may be adjusted by suitable setting ofthe counting devices and timers on the control panel. These adjustmentsare made in any suitable way for example, by opening a transparent coveron the front of the control panel and once set the controls may belocked to prevent unauthorised further adjustment. Similarly since allthe counting devices and timers are automatically re-set after use, aprogramme for a particular batch size of a particular diameter tabletmay be preset initially and locked in for as long as that product is online. Changing the product may require a new programme to be preset.

The control panel is provided with indicator lamps (not shown) which areilluminated to show which channel is in operation and which stage of thecycle is in progress.

The control unit 6 includes an electronic logic circuit 19, a manualcounting device re-set 20 and a start switch 21.

The control unit also includes an alarm or back-up timer 22 which ispermanently preset for a desired time during which a single cycle shouldbe completed. If this cycle is not completed within this pre-set time,an alarm 23 is triggered to call attention to some fault present. Thisalarm 23 can be either visual or audible and if desired can be arrangedto shut down the entire apparatus.

What is claimed is:
 1. In a method of coating tablets comprising thesteps of tumbling a batch of tablets in a rotating coating pan andsubjecting the tablets while they are in the pan to a plurality oftreatment steps each of which consists of a plurality of treatmentcycles each including a dosing period in which a measured quantity oftablet coating liquid is introduced into the pan, a drying period inwhich drying air is supplied to the interior of the pan and anintermediate mixing period during which the tablets tumble in contactwith the already introduced coating liquid for a predetermined time andno drying air is supplied to the pan, and sensing the temperature of thebatch of tablets being coated in the coating pan during the dryingperiod, the improvement comprising preselecting a time interval thatends during the drying period and storing in a memory device as areference temperature the temperature of the batch of tablets at the endof the preselected time interval from the commencement of the dryingperiod, continuing the sensing of the temperature of the batch oftablets in the pan and producing a signal whenever during a dryingperiod there is a preselected difference between the sensed temperatureof tablets in the pan and the reference temperature stored in the memorydevices, and utilizing such signal to terminate the drying period ofsuch cycle and to initiate the dosing period of the next cycle.